Breathing is a remarkable behavior that mediates gas exchange to support metabolism and regulate pH. A reliable and robust rhythm is essential for breathing in mammals. The breathing rhythm consists of two phases of airflow: inspiration and expiration. Failure to maintain a normal breathing rhythm in humans suffering from sleep apnea, apnea of prematurity, congenital central hypoventilation syndrome, hyperventilation syndrome, Rett syndrome, and perhaps sudden infant death syndrome, leads to serious adverse health consequences, even death. Various neurodegenerative diseases, such as Parkinson's disease, multiple systems atrophy and amyotrophic lateral sclerosis, are associated with sleep disordered breathing that we hypothesize results from the loss of neurons in brain areas controlling respiration. If breathing is to be understood in normal and in pathological conditions, the site(s) and mechanisms for respiratory rhythmogenesis must be revealed. Studies of mechanisms underlying rhythm generation have focused primarily on the inspiratory phase while the expiratory period and its specific modulation during passive expiration at rest and active expiration in exercise have been overlooked. We focus on two brain areas essential for breathing, the preBotzinger Complex and the parafacial respiratory group, and propose 2 SPECIFIC AIMS related to the expiratory phase that exploit validated and powerful in vitro models of breathing to advance our understanding of respiratory rhythmogenesis. AIM A: At rest inspiration is active while expiration is passive, suggesting that the quiescent period of the inspiratory rhythm generator defines expiratory duration. Significantly, small changes in respiratory frequency from rest are primarily the result of changes in expiratory, but not inspiratory, duration. We will determine synaptic and cellular mechanisms that control expiratory duration while expiration is passive. AIM B: Active expiration is essential at higher levels of ventilation, such as during exercise. We hypothesize that two rhythmogenic populations, the preBotzinger Complex and the parafacial respiratory group, respectively produce inspiratory and expiratory activity. We will determine the interactions and coordination between these two populations. Breathing is a rhythmic behavior consisting of inspiration and expiration. By detailing how synaptic and cell- intrinsic mechanisms affect expiratory duration at rest, and how two rhythmogenic centers interact when expiration is active, we will significantly improve our knowledge of neural control over the entire breathing cycle. These studies should make fundamental contributions to our understanding o breathing in humans in health and disease. PUBLIC HEALTH RELEVANCE In humans, continuous breathing from birth is essential to life and requires that the nervous system generate a reliable and robust rhythm that drives inspiratory and expiratory muscles. The proposed studies will significantly advance our understanding of the neural mechanisms generating respiratory rhythm and shed light on human disorders of breathing.